Chemical methods sample preparation

The ROPs include sampling, sample preparation, and analysis instructions for low-volume Tenax and XAD-2 air samples. Only the preparation of an XAD-2 low-volume air sample is presented in this article, while the thermal desorption of a Tenax tube is described in the context of gas chromatographic analysis see Chapter 10). Active charcoal is such a strong adsorbent that it requires more effective extraction methods than XAD-2 resin or Tenax tubes. Thus, the recoveries of CWC-related chemicals tend to be lower from active charcoal than from other air sampling materials. Furthermore, active charcoal is not usually used for the collection of organophosphorus chemicals. The sample preparation methods for active charcoal samples have not been validated in international round-robin or proficiency tests. 

Assay methods using combined gas chromatography-mass spectrometry with selected ion monitoring and addition of stable isotopes as carriers and internal standards have been published for the different HETE isomers [208,236,385]. However, so far no GC-MS-assay for LTB4 and related dihydroxy acids or the cysteinyl-containing leukotrienes have been described. Deuterated standards of HETEs were prepared from octadeuterated arachidonic acid either by biosynthesis in mammalian tissues [236,385] or by chemical methods [208]. Preparation of internal standards by chemical methods resulted in deuterated standards with minimal contamination of unlabeled material. This resulted in assays with a detection limit of about 1 pmol for the different HETEs. A GC-MS assay without addition of deuterated internal standards has also been described for analysis of the ratio between different HETEs in the same sample [210]. 

One of the exceptional advantages of the isotope dilution technique compared with other methods is that a ratio (isotope ratio, R) is measured, whereas in all other analytical methods the amount of analyte needs to be measured, such that non-quantitative recovery during the sample pretreatment needs to be corrected for. For IDMS analysis, on the other hand, once the isotope dilution step has taken place, the recovery does not need to be known. This is because, for example, subsequent chemical separation procedures will usually not change the isotope ratio of the isotope-diluted sample to a significant extent. From this, it follows that sample loss does not affect the analytical result in IDMS analysis. An important precondition for successful element determination via isotope dilution is an equilibration of the sample and spike as early as possible in the analytical procedure. This prerequisite is best fulfilled in the case of solutions. Solid samples therefore need to be completely digested and thereby equilibrated with the spike, usually added in the form of an aqueous solution. If the analyte and the spike in the isotope-diluted solution are not equilibrated and exist in different chemical forms, sample preparation steps such as chromatographic separation will fractionate the analyte from the spike and the corresponding isotope ratio measurements will produce incorrect results. 

A number of different sample preparation procedures will now be described to illustrate how the appropriate method will vary, both with the physical nature of the sample, and the chemical character of the components of interest. The examples have been taken from a variety of sources, including application notes from the manufacturers of stationary phases and different chromatography journals. 

Sample preparation techniques vary depending on the analyte and the matrix. An advantage of immunoassays is that less sample preparation is often needed prior to analysis. Because the ELISA is conducted in an aqueous system, aqueous samples such as groundwater may be analyzed directly in the immunoassay or following dilution in a buffer solution. For soil, plant material or complex water samples (e.g., sewage effluent), the analyte must be extracted from the matrix. The extraction method must meet performance criteria such as recovery, reproducibility and ruggedness, and ultimately the analyte must be in a solution that is aqueous or in a water-miscible solvent. For chemical analytes such as pesticides, a simple extraction with methanol may be suitable. At the other extreme, multiple extractions, column cleanup and finally solvent exchange may be necessary to extract the analyte into a solution that is free of matrix interference. 

In most alpha and mass spectrometric methods for which sample preparation is extensive and chemical recoveries can vary considerably from sample to sample, precise elemental concentrations are determined by isotope dilution methods (e.g., Faure 1977). This method is based on the determination of the isotopic composition of an element in a mixture of a known quantity of a tracer with an unknown quantity of the normal element. The tracer is a solution containing a known concentration of a particular element or elements for which isotopic composition has been changed by enrichment of one or more of its isotopes. 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Analyte dilution sacrifices sensitivity. Matrix matching can only be applied for simple matrices, but is clearly not applicable for complex matrices of varying composition. Accurate correction for matrix effect is possible only if the IS is chosen with a mass number as close as possible to that of the analyte elements). Standard addition of a known amount of the element(s) of interest is a safe method for samples of unknown composition and thus unknown matrix effect. Chemical separations avoid spectral interference and allow preconcentration of the analyte elements. Sampling and sample preparation have recently been reviewed [4]. 

Destructive solid sample preparation methods, such as digestion and mineralisation, are well known as they have been around for some time they are relatively cheap and well documented [13-15]. Decomposition of a substance or a mixture of substances does not refer so much to the dissolution, but rather to the conversion of slightly soluble substances into acid- or water-soluble (ionogenic) compounds (chemical dissolution). 

The results for bacterial whole-cell analysis described here establish the utility of MALDI-FTMS for mass spectral analysis of whole-cell bacteria and (potentially) more complex single-celled organisms. The use of MALDI-measured accurate mass values combined with mass defect plots is rapid, accurate, and simpler in sample preparation then conventional liquid chromatographic methods for bacterial lipid analysis. Intact cell MALDI-FTMS bacterial lipid characterization complements the use of proteomics profiling by mass spectrometry because it relies on accurate mass measurements of chemical species that are not subject to posttranslational modification or proteolytic degradation. 

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